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jQuery is a small JavaScript library commonly used to simplify working
with the DOM and JavaScript in general. It is the perfect tool to make
web applications more dynamic by exchanging JSON between server and
client.

JSON itself is a very lightweight transport format, very similar to how
Python primitives (numbers, strings, dicts and lists) look like which is
widely supported and very easy to parse. It became popular a few years
ago and quickly replaced XML as transport format in web applications.

In order to use jQuery, you have to download it first and place it in the
static folder of your application and then ensure it’s loaded. Ideally
you have a layout template that is used for all pages where you just have
to add a script statement to the bottom of your <body> to load jQuery:

In this case you have to put jQuery into your static folder as a fallback, but it will
first try to load it directly from Google. This has the advantage that your
website will probably load faster for users if they went to at least one
other website before using the same jQuery version from Google because it
will already be in the browser cache.

Do you know where your application is? If you are developing the answer
is quite simple: it’s on localhost port something and directly on the root
of that server. But what if you later decide to move your application to
a different location? For example to http://example.com/myapp? On
the server side this never was a problem because we were using the handy
url_for() function that could answer that question for
us, but if we are using jQuery we should not hardcode the path to
the application but make that dynamic, so how can we do that?

A simple method would be to add a script tag to our page that sets a
global variable to the prefix to the root of the application. Something
like this:

The |safe is necessary in Flask before 0.10 so that Jinja does not
escape the JSON encoded string with HTML rules. Usually this would be
necessary, but we are inside a script block here where different rules
apply.

Information for Pros

In HTML the script tag is declared CDATA which means that entities
will not be parsed. Everything until </script> is handled as script.
This also means that there must never be any </ between the script
tags. |tojson is kind enough to do the right thing here and
escape slashes for you ({{"</script>"|tojson|safe}} is rendered as
"<\/script>").

In Flask 0.10 it goes a step further and escapes all HTML tags with
unicode escapes. This makes it possible for Flask to automatically
mark the result as HTML safe.

Now let’s create a server side function that accepts two URL arguments of
numbers which should be added together and then sent back to the
application in a JSON object. This is a really ridiculous example and is
something you usually would do on the client side alone, but a simple
example that shows how you would use jQuery and Flask nonetheless:

As you can see I also added an index method here that renders a
template. This template will load jQuery as above and have a little form
we can add two numbers and a link to trigger the function on the server
side.

Note that we are using the get() method here
which will never fail. If the key is missing a default value (here 0)
is returned. Furthermore it can convert values to a specific type (like
in our case int). This is especially handy for code that is
triggered by a script (APIs, JavaScript etc.) because you don’t need
special error reporting in that case.

Your index.html template either has to extend a layout.html template with
jQuery loaded and the $SCRIPT_ROOT variable set, or do that on the top.
Here’s the HTML code needed for our little application (index.html).
Notice that we also drop the script directly into the HTML here. It is
usually a better idea to have that in a separate script file:

I won’t go into detail here about how jQuery works, just a very quick
explanation of the little bit of code above:

$(function(){...}) specifies code that should run once the
browser is done loading the basic parts of the page.

$('selector') selects an element and lets you operate on it.

element.bind('event',func) specifies a function that should run
when the user clicked on the element. If that function returns
false, the default behavior will not kick in (in this case, navigate
to the # URL).

$.getJSON(url,data,func) sends a GET request to url and will
send the contents of the data object as query parameters. Once the
data arrived, it will call the given function with the return value as
argument. Note that we can use the $SCRIPT_ROOT variable here that
we set earlier.